PROJECT SUMMARY/ABSTRACT There is an abundance of evidence demonstrating the importance of Fc-mediated effector function to the overall activity of an antibody in vivo. However, the relative contribution of virus neutralization versus effector functions to the antiviral effect of an antibody remains undefined. We have proposed a series of experiments that will quantify the contributions of Fc-mediated effector functions to the overall activity of an antibody. The quantitative experiments will be performed in the setting of antibody treatment of SHIV infection in rhesus macaques. These fundamental questions in immunology have yet to be answered, and the resultant information promises to provide important insights on how the two major properties (neutralization versus effector functions) of a vaccine- induced IgG response combine forces to ward off the establishment of HIV infection.